The long-term goal is to understand the cellular and molecular mechanisms linking visual experience during early life to maturation of excitatory synapses in the visual cortex. The maturation of cortical synapses can be detected biophysically by a switch from EPSP depression following repetitive presynaptic action potentials (immature) to EPSP facilitation (more mature). The general working hypothesis is that the molecular composition of postsynaptic structures correlates with and confers the degree of maturity upon the presynaptic axons. We will test this hypothesis by combining patch-recording of multiple, synaptically connected neurons within neonatal rat visual cortical slices with electron microscopic (EM) - immuno- cytochemical (ICC) analysis of the recorded neurons to DETERMINE WHETHER: (1) the more mature, facilitating synapses exhibit the NMDA receptor (NMDAR) subunits - NR1, NR2A - at postsynaptic densities, as well as the muscarinic acetylcholinergic receptors (mAChR) pen-synaptically; (2) the immature, depressing synapses are characterized by pioneer NMDARs that arrive to the plasma membrane first, along with alpha7 nicotinic AChR; (3) activation of these 'pioneer' NMDARs regulate recruitment of cytoplasmic NMDAR subunits and mAChRs to nascent postsynaptic sites; (4) pharmacological blockade of NMDAR will prevent the insertion of NR1/NR2A heteromers of NMDAR and AChR and also delay or abolish the switch at synapses from the depressing to the facilitating phenotype. The works of Aoki and Reyes indicate that synapse maturity can vary widely within single layers and even within single neurons. Thus, the combined EM, ICC and biophysical analysis of single synapses and single postsynaptic densities should be particularly helpful in elucidating functional links between ultrastructure, molecular composition, and physiological properties of excitatory synapses that form during early postnatal life in the visual cortex and dictate life-long capacities of cortical neural function. The knowledge gained from such a study is required in designing molecular remedies for deficits caused by sensory deprivation during early life.